This invention concerns a system and method for making composite products, including board products, by applying a hot, dry gas to filler and thermoactive materials, particularly cellulosic and thermoplastic materials.
Products that combine wood materials with thermoplastic materials are known. Recently, products comprising waste plastics and waste cellulosic materials have been developed. Most of these known products are made by extrusion or injection-die methods, or processes that use only heated platens to apply heat and a compression force to the substrate. Partially or fully continuous processes would be preferred.
A Examples of patented inventions concerning wood/plastic composite products include:
(a) Smith""s U.S. Pat. No. 3,995,980, which describes forming mixtures of materials using three separate delivery systems, and thereafter extruding products comprising the mixture;
(b) Goforth et al.""s U.S. Pat. No. 5,088,910, which describes an extrusion process for making synthetic wood products from recycled materials, such as low or high density polyethylene;
(c) Wold""s U.S. Pat. No. 5,435,954, which discusses a method for forming wood-plastic composites comprising placing mixtures of such materials in molds and subjecting the mixture to sufficient temperatures to cause the material to occupy the mold and assume its shape; and
(d) Reetz"" U.S. Pat. Nos. 5,155,146 and 5,356,278, incorporated herein by reference, which describe extrusion apparatuses and processes for processing charges that include expanded thermoplastic materials, such as polystyrene.
There are several disadvantages associated with the inventions discussed above. A principal problem associated with extrusion and injection methods is that the particle size of the materials used to form the composite must be fairly small. Otherwise, the particle size of the composite mixture is too high to be extruded or injection molded efficiently. Particle size can influence the physical characteristics of productsxe2x80x94if the particle size is too small, then physical properties, such as strength, may be inadequate.
Extrusion and injection processes are further limited by the ratio of filler materials, such as wood, to the thermoactive materials that can be used in the charge (i.e., the mixture of filler material and thermoactive material used to form the final product). This puts undesirable constraints on the products that can be produced.
Another problem associated with prior processes and apparatuses involving heated platens is that they produce products in an entirely batchwise manner, instead of continuously. This substantially reduces product throughput. For example, applying heat to wood and plastic mixtures solely using heated platens takes too long to heat composites completely throughout their cross section. If the temperature of the platens is increased in an effort to speed production, the composite product may burn or scorch, particularly at temperatures above about 400xc2x0 F. Moreover, many processes that use platen presses require that the platen not only be heated but also cooled during each production cycle. This decreases product throughput and is expensive in view of the energy required to complete the serial heating and cooling steps.
Steam injection processes also can be used to produce composites. An initial steam heating stage is followed by continued heating to remove the water applied to the composite during the steam injection process. The combination of steam heating the composite to form products, followed by continued heating to remove water, requires a longer production period relative to processes that do not use steam and is more expensive than is desirable.
Wood-plastic composites have been patented in countries other than the United States. German Patent No. 14 53 374 (the ""374 patent) describes a continuous process for forming composites comprising waste plastic and waste wood. A mixture of waste plastic and waste wood is pressed in the nip between two rollers and hot air is applied to the substrate as it travels around the rollers. The structural features of the apparatus described in the ""374 patent are limiting. For example, the ""374 patent teaches applying hot gas to only one of the two major opposed surfaces of a substrate at a time. Hot air is applied to one surface of a continuous mat as it passes over a first roller. Then, as the continuous mat passes over a second roller, hot gas is applied to the opposite surface of the mat. Considerable energy loss, and therefore added expense, occurs because heated gas is vented to the atmosphere after passing through the composite. The processes described in the ""374 patent, apparently developed in the 1960s, likely could not be used in a current commercial process because the vented gas would include volatile organic compounds (VOCs) that present environmental and health risks.
Furthermore, the process described in the ""374 patent requires that the mat follow the radius of the rollers, which causes the outside surface of the mat to stretch relative to the inside surface. This produces fissures on the outside surface which are undesirable when manufacturing a thick product in combination with an outside surface comprising small particles.
RU 495213 also describes a wood-plastic composite product. Other than stating that the wood fibers in the product may be aligned, RU 495213 provides little description of the composite product, or the apparatus and process used to make the product.
Despite the inventions discussed above, there still is a need for an effective and efficient apparatus and method for continuously forming composite products.
The present invention overcomes the difficulties of the prior art by providing an effective and efficient composite consolidation apparatus and method for continuously applying heat to composite products, or portions thereof, comprising filler materials and thermoactive materials. The apparatus and method are particularly suited for forming composites comprising waste cellulosic materials and waste thermoplastics.
One embodiment of the consolidation apparatus includes a hot-gas distribution system having at least one pair of gas cells, more typically plural paired gas cells, such as rollers or hoods, for applying hot air to the charge. A first cell of each pair applies gas to the charge, and generally is referred to as an application cell. The second cell of each pair operates at a pressure less than the application cell, i.e., a pressure differential exists between the application cell and the suction cell. Certain embodiments of the apparatus include at least one set of baffles positioned adjacent a cell, at least one shroud positioned about a cell, or at least one set of baffles positioned adjacent a first cell and at least one shroud positioned about a second cell to eliminate or substantially reduce the amount of gas that is vented to the surrounding atmosphere.
The consolidation apparatus can be used in combination with various other apparatuses to form systems for forming different embodiments of filler material/thermoactive material composite products. For example, embodiments of such systems can comprise: (1) a mixer for continuous or batchwise formation of mixtures of filler material and thermoactive material; (2) a mat-forming apparatus and an aligner which aligns and graduates filler materials; (3) optionally a prepress for optional densification of the mixture prior to subsequent treatment; (4) a consolidation apparatus having a thermal consolidation zone, and perhaps a densifying zone, for continuously applying hot gas to a moving charge, the zone having at least one gas cell, and perhaps plural gas cells, for applying gas to the moving charge; (5) a mechanical densifying apparatus for applying a densifying pressure to the charge downstream of the consolidation zone; (6) a device for if applying materials, such as plastic films, to composite products; (7) a surface modifying apparatus; (8) a paint station; and various combinations thereof.
The invention further comprises a method for continuously forming composites. A mixture is formed comprising a thermoactive material and a filler material. The mixture is then continuously consolidated by applying a hot, dry noncondensable gas to the mixture. The apparatus described above may be used to continuously apply the gas to the mixture, and the mixture may move continuously through a zone where the consolidating gas is applied. Generally, but not necessarily, the filler material comprises cellulosic material, and the thermoactive material is a thermoplastic material. The mixture may further include materials selected from the group consisting of preservatives, biocides, fungicides (such as metal borate salts, including zinc borate, which typically is used in amounts of from about 0.2% to about 1% by weight of wood), retardants, conductive materials (such as carbon black), pigments, water retardants, wax-like materials (such as Borden""s EWS 404 wax emulsion, which can be used to help bind materials, such as borate-salt fungicides, to the composite), coupling agents (which enhance the interaction between the filler material and the thermoactive material), crosslinking agents (which are used to form inter- or intramolecular bonds between thermoactive materials), materials added for aesthetic purposes (such as dyes), continuous or non-continuous reinforcing materials (such as fiberglass, carbon fibers and Kevlar(copyright)), metal mesh or woven materials (which can be placed within or on the surfaces of composite products), abrasion resistant materials, impact resistant materials, and combinations thereof.
An embodiment of the method can involve forming a mat from a mixture comprising a predetermined amount of cellulosic material and a predetermined amount of thermoplastic material, consolidating the mat and pressing it to a predetermined final product thickness. At least a portion of a surface of the mat or product is then surface modified. Consolidating the mixture into a mat can include forming a first face using a face mixture comprising cellulosic fines, forming a core on the first face using a core mixture, and forming a second face on the core using the face mixture. To form a face, a face mixture is made comprising a predetermined amount of thermoplastic material and cellulosic material. The mixture is deposited on a forming surface so that a wood-rich surface is adjacent the forming surface, which substantially facilitates release of the face from the forming surface. Heat energy is then applied to the mixture to form a melted face layer. Forming the core may include depositing mixtures of wood flakes and plastic flakes according to mixture flake size such that the core has larger flakes adjacent the first face and adjacent the second face, and smaller flakes between the larger flakes in a middle portion of the core. The method also can include fusing a sheet comprising a plastic material to least one major planar surface of the mat. The plastic material at a surface of the composite can be surface modified, such as by e-beam or flame treating the material. Grafting chemicals can be applied to surface modified mats, plastic material or products. Once consolidated, the final composite product comprises a continuous matrix of plastic material having cellulosic fines and flakes of varying sizes.
Various products can be made by varying the number of faces, cores, or both, or by adhering two separate products together. For example, products can be made having just a first face and a core. The method comprises preforming a first face, and depositing a core on an upper surface of the preformed first face.
A composite product also can be made that includes a first portion having a first core, and perhaps a face, formed from mixtures comprising cellulosic material and a thermoplastic material. The product also comprises a second portion bonded to the first portion. The second portion comprises a second core formed from a mixture comprising a cellulosic material and a thermoplastic material, and at least one face attached to the second core.